1. Field of the Invention
This invention relates to gas meters and, particularly, temperature compensated gas meters which provide an accurate indication of energy delivered to the customer throughout wide variations in the temperature of the gas being metered.
2. Description of the Prior Art
It is well known in the gas metering art that the energy content of a given volume of gas will vary significantly with changes in the temperature of the gas. As the temperature rises, the gas expands and the energy content or heating value of a one cubic foot of gas will be less at 80.degree. F. than it is at 60.degree. F. Since residential gas meters are often times installed outside of the residence, they are exposed to wide variations of ambient temperature which would cause significant variations in the energy content delivered to the customer for a given volume of gas registered on the meter. Thus, it has become an established practice in the art to include compensating means in the meter which provides an accurate indication of energy delivered to the customer throughout wide variations in the ambient temperature.
This may be done in either one of two different ways; by adjusting the displacement of the meter or the amount of gas passed through it for a given volumetric reading on the register, or by adjusting the reading on the register for a given amount of gas passed through the meter. This invention is concerned with the former approach.
A conventional domestic gas meter is disclosed in U.S. Pat. No. 2,544,655 to E. R. Gilmore. In such meters, the gas passes through expansible bellows or diaphragm chambers, the movable walls of which are connected to flag rods oscillated or rocked back and forth about their axes by the motion of the diaphragms. The flag rods, in turn, are connected through linkages to a tangent post on a tangent arm which rotates a drive shaft to drive the volume indicating register and to drive the control valves for admitting and discharging gas to and from the diaphragm chambers. The tangent arm is a crank which drives a drive shaft about a fixed axis and the effective length from the axis of the drive shaft to the tangent post where it is connected to the flag rod linkage is usually manually adjustable to provide means for calibrating the meters at assembly. The length of the tangent arm determines the arc of oscillation of the flag rods which, in turn, controls the stroke of the bellows or diaphragms and, therefore, the volume of gas passed through the meter for each revolution of the crank arm about the axis of the drive shaft. Thus, if the radius of the circular path through which the tangent post orbits is varied in accordance with temperature changes, the amount of gas passed through the meter for a one revolution of the tangent arm and tangent post will be varied.
Various means for automatically adjusting the radius of the orbital path of movement of the tangent post in accordance with temperature changes have been proposed in the prior art. All of the following patents show typical compensating arrangements and all employ bimetallic elements as the temperature responsive elements.
U.S. Pat. Nos. 3,119,264 and 3,132,513 both show two bimetallic elements connected in parallel between the tangent post and the valve drive shaft, the bimetals being responsive to temperature changes to automatically vary the distance between the tangent post and the valve drive shaft.
U.S. Pat. Nos. 2,912,859 and 4,040,296 show nested, multiple, parallel bimetals to provide rigidity in the drive from the tangent post to the valve control shaft.
U.S. Pat. Nos. 2,778,224; 2,753,712; 3,069,927 and 3,253,464 show various arrangements of bimetallic temperature sensing elements to vary the distance between the tangent post and the axis of the control shaft.
U.S. Pat. Nos. 3,177,713; 3,177,714; 3,362,223 and 3,371,532 all show bimetallic temperature compensating devices for three chamber gas meters where the temperature compensating device is located in the chamber between the two diaphragms.
All of the above listed patents disclose effective means for adjusting the radius of the orbital path of movement of the tangent post about the axis of the drive shaft. However, in each case, the proposed arrangement is either unnecessarily complicated, expensive to manufacture or presents other problems. For example, it is highly desirable to maintain as linear as possible the radial path of movement along which the tangent post is adjusted toward and away from the axis about which it rotates. In U.S. Pat. Nos. 3,119,264 and 3,132,513 this is done by providing two essentially complete bimetallic loops in parallel with each other so that any tendency for the travel imparted by one loop to depart from a linear radial path is opposed by the other parallel loop.
U.S. Pat. Nos. 3,069,927; 3,253,464 and 4,040,296 show rather complicated devices in which the motion of the element bearing the tangent post is guided to insure linearity of movement. All of these prior art devices, while reasonably effective to achieve their basic purpose for temperature compensation, nevertheless, present complicated and relatively expensive devices for doing so. U.S. Pat. No. 2,368,289 shows an instrument for an automobile panel comprising two opposite acting bimetals which are serially connected. The first bimetal is used to sense changes in the engine temperature or other parameter to be measured, the second bimetal being employed to produce an adjustment for changes in ambient temperature. There is no teaching of linearity of movement of the free end of the sensing element and the movement of the free end is, in fact, non-linear.
U.S. Pat. No. 1,848,952 shows a similar automobile instrument and discloses bourdon tube and a bimetal in series, the bourdon tube sensing the temperature of the engine oil. The function of the bimetal is not clearly disclosed but, apparently, it compensates for changes in ambient temperature in the same manner as disclosed in U.S. Pat. No. 2,368,289.
U.S. Pat. No. 2,043,834 discloses two bimetals connected to a rod in opposition to each other. However, the bimetals are not connected between a fixed and linearly movable point as is required in a temperature compensator for a gas meter.